1. Field of the Invention
This invention relates to systems for controlling the power applied to a load. The load may be resistance or reactive. Applications of the invention include audio amplifiers and motor controllers.
2. Discussion of the Prior Art
Power controlling systems have wide spread utility. One of the more extensive uses is in power amplifiers, particularly audio amplifiers. Traditionally, power controllers utilized linear techniques such as class A amplifiers, class B amplifiers, and class AB amplifiers. The major drawback of these amplifiers has been low efficiency. The greatest theoretical efficiency of a linear amplifier is 78.5 per cent for a class B amplifier, but as a practical matter, the best linear amplifiers can only obtain 50 per cent efficiency and usually operate with about 30 per cent efficiency. Low efficiency means that, in order to obtain an output of a load of 30 watts, it is necessary to provide a driving power source of at least 100 watts. This means 70 watts of power is dissipated, principally in the form of heat, by the power controller. The greater the need for power, such as audio power, the greater the need for proper ventilation, the greater the need for heat sinks to dissipate that heat, and the greater the need for more expensive components that can withstand the power, as heat, being dissipated in those components.
The foregoing becomes a problem, for example, in the installation of audio power amplifiers in automobiles. Installation of high power audio amplifiers in the dashboards of automobiles becomes a problem due to a lack of ventilation and a lack of room for large heat sinks. Therefore, manufacturers find it necessary to install the power amplifiers in the trunks of automobiles, installing only the tuner, preamplifier, cassette player, etc. in the dashboard. Accordingly, there is a need for a light weight, high-efficiency audio power amplifier for use in automobiles, an amplifier that can be installed entirely in the dashboard of the automobile along with the radio.
Other advantages of high efficiency include a reduction in the cost of building the power controller, since the heat sinks can be reduced in size and weight, since the electronic components such as transistors can be replaced with lower powered devices, and since the power demand on the power source for the power controller can be reduced.
A variety of non-linear techniques have been utilized to greatly increase the efficiency of power controllers, such as audio amplifiers. These amplifiers take on many forms utilizing modulation techniques. For example, U.S. Pat. No. 4,178,556 discloses a class D amplifier system including a pulse width modulator and U.S. Pat. No. 4,500,844 discloses a switching amplifier utilizing delta-modulation techniques. These power controlling techniques achieve high efficiency by utilizing power controlling devices such as transistors that operate as switches instead of linearly. A transistor operating as a switch dissipates no power when it is off and dissipates very little heat when it is on because of its low on-resistance and low voltage drop across the device.
These power controllers which utilize modulation or switching techniques have analog outputs which have limited linearities and limited signal-to-noise ratios. One method to increase the linearly or signal-to-noise ratio is to increase the frequency of the modulation or switching. There is a practical limit to higher frequency switching because of the limitation of the switching speed of the power switching devices, typically transistors. Another method is to improve the low-pass filter that is used to attenuate the error components outside the band of interest. Traditionally, the limit on the low-pass filter is a second order filter, because a higher order filter will cause instability in a closed loop system.
The present invention includes the use of a third or higher order filter in the closed loop, where the use of such a filter greatly enhances the signal-to-noise ratio and linearity of the analog output. Stability is improved by the use of a filter design which is normally undesirable in that it is underdamped. Underdamped means that the output is inclined to oscillate when stimulated by an impulse or step function at its input. Such underdamped filters have unusual gain and phase responses that are normally considered undesirable, and hence, designers normally will not use them. The only other practical application that uncovered of such a filter is described in U.S. Pat. No. 4,509,037 which covers the invention of an Enhanced Delta Modulation Encoder. This invention describes an analog-to-digital converter which creates a single bit digital data stream representative of the analog signal being measured.
In view of the foregoing, it is an object of the present invention to provide a high-efficiency, power controller with the efficiency being of the order of 90 percent.
It is another object of the present invention to provide a power controller having a high signal-to-noise ratio and high linearity.